U.S. Pat. No. 4,791,268 teaches a contact-start plasma arc torch where the electrode is biased forward by a plunger which resides in an enclosed structure; this structure is formed such that a significant portion of the electrode is not exposed to gas flow which would enhance cooling, and there appears to be little gas flow past the plunger. Additionally, the contact between the plunger and the electrode is provided only across relatively small planar contact surfaces, which may be susceptible to reduced contact due to any unwanted material interposed between these surfaces.
U.S. Pat. Nos. 8,035,055 and 8,115,136 teach a variety of electrode configurations for contact-start plasma arc torches, as well as teaching a prior art electrode which employs a spring-loaded contact for the power supply for biasing the electrode toward its forward position. In the prior art device cited in these patents, the electrode and contact remain engaged at all times. In basic embodiments of the invention taught in these patents, a spring is positioned between the electrode and the contact to bias the electrode away from the contact. In these embodiments, the electrode directly engages the contact only when in its rear position, which is the position for sustaining the non-transferred pilot arc and the transferred cutting plasma arc. Unless the electrode includes the spring, the spring may be lost when the torch is opened to change the electrode.
Perhaps to avoid the possibility of springs being lost when the torch is opened to change the electrode, these patents also teach several embodiments that employ an electrode having a spring-loaded conductive element that is secured to the electrode, trapping the spring. Securing the conductive element and spring to the electrode requires that these components be replaced with the electrode, increasing expense of the electrode, which is a consumable part.
In still other embodiments, the electrode is installed via a bayonet-style connection where the spring is positioned behind the female section of the bayonet element and thus trapped in the assembly. The electrode is provided with the male portion of the connection and, when inserted and locked in position, this portion contacts the spring. In another embodiment, the spring is retained by a fixed ring which overlaps part of the spring and a pair of prongs are positioned so as to pass though the opening and engage the spring. Such a configuration provides limited contact. While these latter solutions overcome the expense of attaching the spring and a conductive element to the electrode, it complicates the structure of the electrode, again increasing expense of fabrication, and may limit air flow over the spring and the contacting portion of the electrode, thereby reducing cooling. These embodiments also appear to suffer from limited engagement between the spring and the electrode, thus limiting the effectiveness of electrical contact therebetween. These limitations may explain why the electrode currently being commercially offered by the patentee is the embodiment shown in FIGS. 3A and 3B of the '155 and '136 patents, which has a spring and a conductive element secured to the electrode.